Geothermal direct expansion heating/cooling systems are generally well known systems which are essentially "heat pumps", transferring heat via a common refrigeration cycle, from one source to another, with at least one of two or more of the system's heat exchange elements being buried in the ground or submersed in water, such as a lake or pond.
As referenced, Geothermal direct expansion heating/cooling systems include at least one heat exchange element, typically consisting of closed loops of tubing, buried in the ground or submersed in water. These closed loops of tubing may be installed in a variety of manners, including horizontal configurations or helical loops, as well as in various vertical configurations, such as spiraled coils or elongated U-shaped tubes. These subterranean tubes, or loops, typically carry a refrigerant, such as R-22, or the like, in direct expansion heating/cooling systems to assist in effecting heat transfer.
Regarding direct expansion subterranean (below ground/below water surface) heat exchange tube sizing, it is well known that refrigerant charge imbalances exist, in all current system designs, between summer and winter seasons. While, ideally, refrigerant charges should be equivalent, this is presently not the case in any known direct expansion system.
Historically, this interior volume imbalance exists between the interior air heat exchange coils and the exterior, subterranean, heat exchange coils. The resulting refrigerant charge imbalance has been due to three primary reasons: (1) the perceived notion that larger diameter subterranean tubes provide greater heat transfer capacity than smaller diameter tubing, which could be correct when solely evaluating a tube comparison of an equal number of tubes with equal lengths but with varying interior diameters; (2) the perceived notion that more tubing is required below ground in subterranean finnless heat exchange tubing than above ground in the fan assisted air handler containing finned tubing, which could be correct with conventionally designed systems since subterranean tubing must be spaced over an adequate minimum ground area, and therefore cannot normally utilize finned tubing, which could concentrate heat exchange in too small an area to effectively achieve the necessary seasonal overall system heat exchange operational efficiencies; and (3) the perceived notion that the less the number of tubes in the subterranean heat exchanger, the faster the installation, and the lower the initial installation cost, which is generally correct with most direct expansion ground coil design layouts.
As a result, prior art has developed the following ground tube size ratios:
In DE3514191A1 to Waterkote, the use of direct expansion geothermal heat transfer tubing with internal diameter to length ratios of between 1/2857 and 1/3750 was taught.
Aardvark Air, Inc., formerly of 700 Prospect, Kansas City, Mo. 64132, reportedly sold and installed direct expansion systems utilizing 100 foot long, ACR grade (0.03 inch wall thickness), 1/4 inch outside diameter copper tubes for the subterranean heat exchanger. These tubes had an external diameter to length ratio of 1/4,800, which ratio was taught and later claimed as proprietary by U.S. Pat. No. 5,025,634 to Dressler, as subsequently assigned to USPower Climate Control, Inc., which manufactured and sold direct expansion systems utilizing the said 1/4,800 ratio.
Further, USPower Climate Control, Inc., formerly of 954 Marion Blvd., Allentown, Pa. 18103, during or about 1990 through 1991, also sold direct expansion systems utilizing 100 foot long, ACR grade (0.032 inch wall thickness), 5/16 inch outside diameter, copper tubes for its subterranean heat exchangers. These tubes had an external diameter to length ratio of 1/3,840.
All of the above-referenced geothermal direct expansion ground coil designs utilize 1/4 inch outside diameter, or larger, subterranean heat exchange tubing, which is commonplace among all known direct expansion systems. Further, while the subterranean tube surface area per ton (per 12,000 BTUs) of heating/cooling system design capacity is uncertain in Waterkote's design, all other above-referenced designs utilize a combined tube surface area of about 32.6 square feet to 39.2 square feet per ton of design capacity, which is typically exposed, via a network of approximately equally spaced and arrayed tubing, to a ground surface area of about 500 square feet per ton of design capacity.
Further, in order to combat the aforesaid charge imbalance resulting from the differing interior volume areas in the interior air handler heat exchange coils versus the exterior subterranean heat exchange ground coils, the use of a receiver has traditionally been employed, so as to automatically hold unnecessary refrigerant in reserve within a holding container on the high pressure side of the compressor.
The charge imbalance results when one switches the system from a heating to a cooling mode, or vise a versa. Typically, in the heating mode, the exterior ground coils are the evaporator section of the system and the interior air handler coils are the condenser section. When the system is switched to a cooling mode, the exterior ground coils become the condenser and the interior air handler coils become the evaporator. Since, with conventional geothermal direct expansion systems, the ground coils combined interior volume is greater than that of the air handler's combined interior volume, more liquid refrigerant is required for efficient system operation in the cooling mode than in the heating mode.
The use of a conventionally sized, or larger, receiver to automatically adjust the aforesaid charge imbalance in direct expansion systems is well known. The use of a conventional receiver was taught, although not claimed as proprietary, by U.S. Pat. No. 4,688,717 to Jungwirth. A conventional receiver was reportedly used by the aforesaid Aardvark Air, Inc., in direct expansion systems during or about 1982. A conventional receiver was taught for use in a direct expansion system via a textbook entitled "Modern Refrigeration and Air Conditioning" by Andrew D. Althouse, Carl H. Turnquist, and Alfred F. Bracciano, published by The Goodheart-Willcox Company, Inc., copyright 1975. The use of an oversized 20% total refrigerant quantity capacity receiver in a direct expansion system was claimed as proprietary by U.S. Pat. No. 5,461,876 to Drlessler. Envirotherm Heating and Cooling, Inc., of 105 Forrest Retreat, Hendersonville, Tenn. 37075, marketed and sold a direct expansion system in 1995 which contained an oversized receiver designed to store close to 50% of the total refrigerant quantity. However, the use of any conventionally sized, or larger, receiver in a direct expansion system mandates a certain equipment manufacturing cost for both materials and labor.
Regardless of the extra equipment and manufacturing cost, standard or oversized receivers are necessary in currently designed direct expansion systems in order to reduce system operational inefficiencies otherwise occurring in the heating and/or cooling mode. In fact, if the system's refrigerant charge volume differential between the interior air handler coil volume and the subterranean ground coil volume was too great, absent a receiver, the system could potentially totally fail to reverse cycle (switch to heating mode from cooling mode, or vice a versa).
Regarding the sizing and matching of system components, conventionally, direct expansion systems have been sized by a determination of the subject structure's heating/cooling load, via ACCA Manual J, or similar, BTU heating/cooling load calculation criteria. Thereafter, the compressor is sized to match the calculated BTU load, where one ton of capacity equals 12,000 BTUs. The air handler is sized to match the capacity of the compressor, so that, for example, a manufacturer's three ton compressor is matched with a manufacturer's three ton air handler. The corresponding ground coils are also conventionally sized by the manufacturer to match the conventional compressor and conventional air handler sizing. A commonly used ground coil design would be five 100 foot long, 1/4 inch diameter, ACR grade, tubes per ton of compressor capacity. Conventionally, an accumulator and a receiver, designed to match the conventionally sized compressor, or an oversized accumulator and an oversized receiver, together with thermal expansion valves, sized to match the compressor tonnage, are also utilized in direct expansion applications. The best method of sizing and matching system components is an area of critical concern for overall highly efficient system operation.
Lastly, none of the above-referenced in-ground/in-water heat exchangers utilize finned tubing, as is commonly utilized in air source heat pump heat exchange units, so as to increase air surface contact area and so as to correspondingly accelerate heat transfer from the refrigerant to the air, or vise a versa. This absence of finned tubing in ground heat exchange tubing is partially because conventionally sized fins surrounding in-ground heat exchange tubing would be anticipated to inhibit full good ground contact, leaving air pockets between the fins, which air pockets would result in thermal transfer inefficiencies. In typical direct expansion applications, where natural earth is utilized as a fill material, this anticipated concern would likely constitute a valid reason not to utilize conventional finned tubing. Conventional finned tubing may consist of tubing with between 8 to 16 fins per inch of tubing length, with fins typically not extending more than one half inch from the exterior perimeter of the tubing.
Another reason finned tubing has not heretofore been utilized in the ground coils of direct expansion heat pump applications is because the earth surrounding the refrigerant laden in-ground heat transfer tubes is limited in its ability to transfer naturally occurring heat to/from the refrigerant within the heat transfer tubes, which tubes are typically constructed of a metal, such as copper. Consequently, the extra cost involved in utilizing standard finned tubing for geothermal direct expansion heat transfer applications has been deemed an unnecessary expenditure. Consequently, conventional direct expansion applications typically rely on a matrix of finnless tubing (typically 1/4 inch to 1/2 inch in outside diameter) to effect the desired heat transfer with the surrounding soil, or ground, which heat transfer into the surrounding ground is sometimes augmented with an artificial fill material placed around the finnless tubing, such as powdered stone, concrete, flowable fill, or the like.
Yet another problem typically encountered with conventional direct expansion systems arises from the use of a single vapor line with a single distribution point, and a single liquid line, with a single distribution point, to and from the interior equipment from and to the multiple, smaller, subterranean heat exchange tubes. Typically, the single vapor and liquid line sets are kept at an equal length, and require a congregation of the multiple subterranean heat exchange tubes at respective single point for the vapor line and for the liquid line, which diminishes ground contact area with naturally occurring geothermal heat. Worse, these congregation points are often in relatively close proximity to one another, and thereby tend to subject the returning heat exchange lines to the extreme heat or cold of the outgoing heat exchange lines, depending on whether operating in a cooling or a heating cycle, thereby negating some of the positively gained geothermal heat exchange effect. A means of avoiding these problems would increase operational efficiencies.
Further, conventionally designed direct expansion ground coils are installed in a large excavated pit, typically requiring a large front end loader excavator, or a large bucket track hoe; in well holes, either vertical or angled, typically requiring a large well drilling rig; in a trench, typically two feet to eight feet wide, requiring a large bucket back hoe, or track hoe; or via a cylindrical design, typically requiring a telephone pole drilling rig, or the like. As a result, conventionally designed direct expansion ground coils are relatively expensive to install, and require a significant amount of earth moving, which is both time-consuming and relatively expensive, as well as being problematic for landscaping when an installation occurs where there is an established lawn. Further, direct expansion ground coils are typically backfilled with earth, which often results in non-heat-conductive air gaps occasioned by unbroken clods of earth surrounding the subterranean heat exchange tubing.
What is needed is an installation method for direct expansion ground coils that is relatively quick, inexpensive, minimally invasive, highly efficient for ground heat transfer, and safe.
Consequently, it is an object of the present invention to provide a more efficient in-ground/in-water heat transfer design than that conventionally utilized in direct expansion applications for either heating/cooling systems or for power generation systems.